(Meth)acrylates are cured by being irradiated with active energy rays such as ultraviolet rays and electron beams or by being heated, and they are thus used in large quantities as a crosslinking component of blended materials such as paints, inks, adhesives, optical lenses, fillers, and molding materials or a reactive diluent component.
In particular, polyfunctional (meth)acrylates having three or more (meth)acryloyl groups are used in large quantities as a blending component of hard coat paints since the cured products thereof exhibit high hardness and excellent abrasion resistance.
As such polyfunctional (meth)acrylates, trimethylolpropane tri(meth)acrylate, glycerol tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, dipentaerythritol tetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, dipentaerythritol hexa (meth)acrylate, tripentaerythritol octa (meth)acrylate, and the like are known.
These (meth)acrylates are produced by an esterification reaction using the corresponding alcohol and the corresponding (meth)acrylic acid as raw materials and sulfonic acid (see Patent Literature 1) as a catalyst or a transesterification reaction using the corresponding alcohol and the corresponding monofunctional (meth)acrylate as raw materials and an organotin compound (see Patent Literature 2) as a catalyst. In addition, with regard to the transesterification reaction, a method is known in which an organophosphorus compound and a zinc compound are concurrently used as a catalyst (see Patent Literature 3).
The catalyst used in the production of these (meth)acrylates is removed by a purification operation after the reaction. In a case in which the catalyst is insufficiently removed, however, the storage stability and thermal stability of the (meth)acrylate obtained are poor, a polymerization reaction and a hydrolysis reaction take place during storage, and a polymer and an acid component such as (meth)acrylic acid are generated in some cases.
Curing unevenness and turbidity of the (meth)acrylate containing a polymer are caused so that the (meth)acrylate cannot be suitably used in the optical lens application in which uniformity and optical transparency are regarded as important. In addition, the (meth)acrylate in which an acid component is generated exhibits deteriorated water resistance in addition to the problems of odor and corrosion of apparatus, and thus the cured product absorbs moisture and peeling of the coated surface and a decrease in adhesive strength are caused in some cases in the case of using the (meth)acrylate in the coating agent and adhesive applications. In addition, the (meth)acrylate is exposed to a heating and stirring treatment for homogenization at the time of blending and a heat resistance test after curing in some cases. However, (meth)acrylic acid esters having poor thermal stability cannot be used at all in the optical lens application and the like which are required to exhibit transparency since coloration of the (meth)acrylic acid esters is caused in addition to the generation of a polymer and an acid component described above.
Patent Literature 1 discloses a method in which the reaction liquid after an esterification reaction using sulfonic acid as a catalyst is subjected to a neutralization treatment and a washing treatment, a quaternary ammonium salt or a quaternary phosphonium salt is further added to the resultant, and a heat treatment is conducted, but the operation is slightly complicated. In addition, Patent Literature 2 discloses a method in which a solvent and the like are added to the resultant after completion of a transesterification reaction using an organotin compound as a catalyst and precipitation and filtration are conducted, but the effect is insufficient and the purified product still contains several hundred ppm of tin.